Remove all the outer leaves from your head of cauliflower and turn upside down on your cutting board. Slice out the core by using a sharp knife and cutting out a cone of it from the middle. Break apart the florets and wash.
Scrub and large dice your potatoes.
Wash your greens and remove the hard stem. Stack all the leaves on top of one another, and roll them up forming one long tube. Finely chop into small ribbons. You can use the cauliflower greens here, too, just prepare as if they were kale or chard. Mince the garlic.
Bring two pots of lightly salted water to a boil, one fitted with a steamer basket the other without. In the steamer basket pot, steam the cauliflower till easily pierced with a knife (7 minutes), and in the other, boil the potatoes till also easily pierced. You could do this all together in one pot, but it goes much quicker in two.
When the veggies are done, drain them or remove with a slotted spoon, but keep one pot boiling (you'll use it in a minute.)
Add the cauliflower, the minced garlic and the potatoes together in one of the now-empty pots, and begin to add in the pre-heated milk or cream. Use a masher, or a ricer, and mash. Leave some chunks for a rustic texture. Taste, add salt and pepper as desired.
Once you've achieved the level of smoothness you prefer, quickly dunk the kale/chard/collard ribbons in the pot that still has boiling water and swish around till soft (1-4 minutes depending on which green you use.) Remove, and mix the greens into the mash, and serve topped with the minced scallions and a few shavings of parmesan. Or, maybe some chopped up and fried bacon, because why not.